Within the past decade, the advent of world-wide electronic communications systems has enhanced the way in which people can send and receive information. Moreover, the capabilities of real-time video and audio systems require a large bandwidth. In order to provide services such as video-on-demand and video conferencing to subscribers, an enormous amount of network bandwidth is required. In fact, network bandwidth is often the main inhibitor to the effectiveness of such systems.
In order to minimize the effects of the constraints imposed by the limited bandwidths of telecommunications networks, compression systems and standards have evolved. These standards prescribe the compression of video and audio data and the delivery of several programs and control data in a single bit stream transmitted in a bandwidth that would heretofore only accommodate one analog program.
One video and audio compression standard is the Moving Picture Experts Group (“MPEG”) standard. Within the <PEG-2 standard, video compression is defined both within a given picture, i.e., spatial compression, and between pictures, i.e., temporal compression. Video compression within a picture is accomplished by conversion of the digital image from the time domain to the frequency domain by a discrete cosine transform, quantization, variable length coding, and Huffman coding. Video compression between pictures is accomplished via a process referred to as motion compensation, in which a motion vector is used to described the translation of a set of picture elements (pels) from one picture to another, Audio compression is as defined in the standard.
The procedure for transporting the compressed bitstream from the transmitting end to the receiving end of the system, and for thereafter decompressing the bitstream at the receiving end, so that one of the many picture sequences is decompressed and may be displayed in real-time is specified in ISO 13818-1. ISO 13818-1 is the systems or transport layer portion of the MPEG-2 standard. This portion of the standard specifies packetization of audio and video elementary bitstreams into packetized elementary stream (PES), and the combination of one or more audio and video packetized elementary stream into a single time division or packet multiplexed bitstream for transmission and the subsequent demultiplexing of the single bitstream into multiple bitstreams for decompression and display. The single time division or packet multiplexed bit stream is as shown from various architectural and logical perspectives in the FIGURES, especially FIGS. 1 to 5, where many packets make up a single bitstream.
The concept of packetization and the mechanism of packet multiplexing are shown in FIG. 1, denominated “Prior Art”, where elementary streams are formed in an audio encoder 101, a video encoder 102, a source 104 of other data, and a source 106 of systems data. These elementary streams are packetized into packetized elementary streams, as described hereinbelow. The packetized elementary streams of audio data, and video data, and the packets of other data and systems data are packet multiplexed by the multiplexor into a system stream.
The time division or packet multiplexed bitstream is shown, for example, in FIGS. 2 and 5, both denominated “Prior Art”, which gives an overview showing the time division or packet multiplexed bitstream. The bitstream is comprised of packets, as shown at 500 in FIG. 5. Each packet, as shown at 200 in FIG. 2, is, in turn, made up of a packet header 201, an optional adaptation field, and packet data bytes, i.e., payload 202.
The MPEG-2 System Layer has the basic task of facilitating and multiplexing of one or more programs made up of related audio and video bitstreams into a single bitstream for transmission through a transmission medium, and thereafter to facilitate the demultiplexing of the single bitstream into separate audio and video program bitstream for decompression while maintaining synchronization. By a “Program” is meant a set of audio and video bitstreams having a common time base and intended to be presented simultaneously. To accomplish this, the System Layer defines the data stream syntax that provides for timing control and the synchronization and interleaving of the video and audio bitstream. The system layer provides capability for (1) video and audio synchronization, (2) stream multiplex, (3) packet and stream identification, (4) error detection, (5) buffer management, (6) random access program insertion, (7) provide data, (8) conditional access, and (9) interoperability with other networks, such as those using asynchronous transfer mode (ATM).
An MPEG-2 bitstream is made up of a system layer and compression layers. Under the MPEG-2 Standard (ISO/IEC 13818-1) a time division of packet multiplexed bit-stream consists of two layers, (1) a compression layer, also referred to as an inner layer, a payload layer, or a data layer, and (2) a system layer, also referred to as an outer layer or a control layer. The compression layer or inner layer contains the data fed to the video and audio decoders, and defines the coded video and audio data stream, while the system layer or outer layer provides the controls for demultiplexing the interleaved compression layers, and in doing so defines the functions necessary for combining the compressed data streams. This is shown in FIG. 3, denominated “Prior Art”. As there shown a bitstream of, for example, a system layer and compression layer, is the input to a system decoder 301. In the system decoder the system layer data is demultiplexed into the compressed audio layer, the compressed video layer, and control data. The control data is shown in FIG. 3, denominated Prior Art, as the PCR (Program Clock Recover) data, enable data, and start up values. The compressed data is sent to the respective audio and video data buffers 302 and 303, and through decoder control 304 to the respective audio and video decoders 305 and 306.
The system layer supports a plurality of basic functions, (1) time division or packet multiplexing and demultiplexing of the time division or packet multiplexed multiple bit-streams, (2) synchronous display of the multiple coded bit stream, (3) buffer management and control, and (4) time recovery and identification. The system layer also supports (5) random access, (6) program insertion, (7) conditional access, and (8) error tracking.
For MPEG-2, the standard specified two types of layer coding, a program stream (PS), for relatively lossless environments, such as CD-ROMs, DVDs, and other storage media, and transport stream (TS), for loss media, as cable television, satellite television, and the like. The transport stream (TS), shown in FIG. 2, denominated Prior Art, consists of a stream of transport stream packets, each of which consists of 188 bytes, divided into 4 bytes of packet header, an optional adaptation field, and up to 184 bytes of the associated packet data, that is, payload. The relationship of the layering of the access units, the PES packets, and the Transport Stream (TS) packets is shown in FIG. 5, denominated Prior Art.
The transport stream (TS) is used to combine programs made up of PES-coded data with one or more independent time bases into a single stream. Note that under the MPEG-2 standard an individual program may not have a unique time base, but that if it does, the time base is the same for all of the elements of the individual program.
The packetized elementary stream (PES) layer is an inner layer portion of the MPEG-2 time division or packet multiplexed stream upon which the transport of program streams are logically constructed. It provides stream specific operations, and supports the following functions: (1) a common base of conversion between program and transport streams, (2) time stamps for video and audio synchronization and associated timing, especially for associated audio and video packets making up a television channel, presentation or program, and having a common time base, (3) stream identification for stream multiplexing and demultiplexing, and (4) such services as scrambling, VCR functions, and private data.
As shown in FIG. 5, denominated Prior Art, video and audio elementary streams (ES) must be PES-packetized before inserting into a transport stream(TS). Elementary streams (ES) are continuous. PES packets containing an elementary stream (ES) are generally of fixed lengths. Typically, video PES packets are in the order of tens of thousands of bytes, and audio PES packets are on the order of thousands of bytes. However, video PES packets can also be specified as of undefined length.
The MPEG-2 packetized elementary stream (PES) packet structure is shown in FIG. 4. To be noted is that all of the fields after the PES packet length 401 are optional. The PES (packetized elementary stream) packet has bit start code 402, a packet length field, a 2 bit “10” field, a scramble control field 405, a priority field, a data alignment field, a copy field, a PTS/DTS (Presentation Time Stamp/Decoding Time Stamp) field, a field for other flags, and a header length field.
The “Optional Header” field 410 includes a Presentation Time Stamp field, a Decoding Time Stamp field, an elementary stream clock reference field, a elementary stream rate field, a trick mode field, a copy info field, a Prior Packetized Elementary Stream Clock Recovery field, an extension and stuffing.
The packet start code provides packet synchronization. The stream ID field 406 provides packet identification. Payload identification is also provided by the stream ID. The PTS/DTS flag fields and the PTS/DTS fields provide presentation synchronization. Data transfer is provided through the packet/header length, payload, and stuffing fields. The scramble control field facilitates payload descrambling, the extension/private flag fields and the provide data fields provide private information transfer.
A transport stream (TS) may contain one or more independent, individual programs, such as individual television channels or television programs, where each individual program can have its own time base, and each stream making up an individual program has its own PID. Each separate individual program has one or more elementary streams (ES) generally having a common time base. To be noted, is that while not illustrated in the FIGURES, different transport streams can be combined into a single system transport stream. Elementary stream (ES) data, that is, access unit (AU), are first encapsulated into packetized elementary stream (PES) packets, which are, in turn, inserted into transfer stream (TS) packets, as shown in FIG. 5, denominated Prior Art.
The architecture of the transport stream (TS) packets under the MPEG-2 specifications is such that the following operations are enabled: (1) demultiplexing and retrieving elementary stream (ES) data from one program within the transport stream, (2) remultiplexing the transport stream with one or more programs into a transport stream (TS) with a single program, (3) extracting transport stream (TS) packets from different transport streams to produce another transport stream (TS) packet into one program and converting it into a program stream (PS) containing the same program, and (5) converting a program stream (PS) into a transport stream (TS) to carry it over a lossy medium to thereafter recover a valid program stream (PS).
At the transport layer, the transport sync byte provides packet synchronization. The Packet Identification (PID) field data provides packet identification, demultiplexing, and sequence integrity data. The PID field is used to collect the packets of a stream and reconstruct the stream. The continuity counters and error indicators provide packet sequence integrity and error detection. The Payload Unit start indicator and Adaption Control are used for payload synchronization, while the Discontinuity Indicator and Program Clock Reference (PCR) fields are used for playback synchronization. The transport scramble control field facilitates payload descrambling. Privite data transfer is accomplished through the Private Data Flag and Private Data Bytes. The Data Bytes are used for private payload data transfer, and the Stuffing Bytes are used to round out a packet.
Achieving and maintaining clock recovery and synchronization is a problem, especially with audio and video bitstreams. The MPEG-2 model assumes an end-to-end constant delay timing model in which all digital image and audio data take exactly the same amount of time to pass through the system from encoder to decoder. The system layer contains timing information that requires constant delay. The clock references are Program Clock Reference (PCR) and the time stamps are the Presentation Time Stamp/Decoding Time Stamp (PTS/DTS).
The decoder employs a local system clock having approximately the same 27 Megahertz frequency as the encoder. However, the decoder clock can not be allowed to free run. This is because it is highly unlikely that frequency of the decoder clock would be exactly the same as the frequency of the encoder clock.
Synchronization of the two clocks is accomplished by the Program Clock Reference (PCR) data field in the packet adaptation field of the PCR PID for the program. The Program Clock Reference values can be used to correct the decoder clock. Program Clock Reference, or PCR, is a 42 bit field. It is coded in two parts, a PCR Base having a 33-bit value in units of 90 kHz, and a PCR extension having a 9-bit extension in units of 27 MHz, where 27 MHz is the system clock frequency.
As a general rule, the first 42 bits of the first PCR received by the decoder initialize the counter in a clock generation, and subsequent PCR values are compared to clock values for fine adjustment. The difference between the PCR and the local clock can be used to drive a voltage controlled oscillator, or a similar device or function, for example, to speed up or slow down the local clock.
Audio and video synchronization is typically accomplished through the Presentation Time Stamp (PTS) inserted in the Packet Elementary Stream (PES) header. The Presentation Time Stamp is a 33-bit value in units of 90 kHz, where 90 kHz is the 27 MHZ system clock divide by 300. The PTS value indicates the time that the presentation unit should be presented to the user.
The system layer timing information, PCR and PTS/DTS, keep the encoder and decoder in synchronization, with the PCR values correcting the decoder clock. The timing information, PCR and PTS/DTS, arrive at the decoder about every 10-100 milliseconds for the PCR, and at least as frequently as about every 700 milliseconds for the PTS/DTS. Processing and filtering the timing signals consumes significant processor resources. This is because the clock signals are in mixed number bases, the clock signals can arrive at widely varying times, there is no way to sort out necessary interrupts from unnecessary interrupts, and, most important of all, errors in clock management are directly visible and/or audible through buffer overflow or underflow and color disturbance. However, as noted above, the relationship between PCR and the STC values are used to drive a voltage controlled oscillator or similar device. The voltage controlled oscillator or similar device speeds up or slows down the local clock driving the STC. In this context, a need exists for functionality in the system to reduce the processing demand on the processor. Specifically, there is a need for (1) reducing the number of clock management interrupts to the processor, and (2) a mechanism to closely match the rates of the encoder and decoder clocks as specified by the PCR and STC values as well as minimizing the difference between the PCR and STC values. The last requirements allows an internal clock recovery mechanism to make small adjustments to the value controlling the local clock frequency without interrupting the processor for a period of time.